1. Field of the Invention
This invention generally relates to an interface circuit for digital signal transmission or communication between separate devices, and specifically relates to an interface circuit for digital audio or video signal transmission between separate devices in an audio system or a video system.
2. Description of the Prior Art
Advanced audio or video systems handle sound or display data in the form of digital signals at certain stages. In these systems, the digital data signals are finally converted into corresponding analog data signals. The deterioration in quality of such a reproduced analog data signal mainly results from the following two causes:
(1) A failure in accurate transmission and digital-to-analog (D/A) conversion of a digital data signal;
(2) Contamination of an analog data signal caused by the ingress of noises ripples and jitters to the circuitry which processes or carries the analog data signal, such noises ripples and jitters are contained in a digital data signal or are generated in a digital signal processing unit.
One known way of removing the previously-mentioned cause (2) of the signal deterioration is to dispose a digital signal processing unit and a D/A converting and analog signal processing unit respectively in separate chassis whereby interference between the two units are minimized. In some cases, these two units in the separate chassis are further housed in a common cabinet. In other cases, they are housed in separate cabinets respectively.
In digital audio systems, another known way of removing the previously-mentioned cause (2) of the signal deterioration is to use a photocoupler in an interface circuit performing digital signal transmission from a digital signal processing unit (a signal transmitting unit) to a D/A converting and analog signal processing unit (a signal receiving unit). In this interface circuit, a digital signal is transmitted between the two units via the photocoupler. The photocoupler enables the two units to remain electro-conductively isolated from each other while signal transmission between them are maintained. According to this electro-conductive isolation, the two units are generally activated by separate power supplies and grounds of the two units are separated also. The electro-conductive isolation is advantageous in that if a ground current of the signal transmitting unit is subject to change because of a change of load to the associated power supply, such variation in the ground current of the signal transmitting unit will not be transmitted to the signal receiving unit. Furthermore, the electro-conductive isolation including the ground isolation is usually effective in cutting off common mode noises.
However, such a known photocoupler-based interface circuit is incapable of removing the previously-mentioned cause (1) of the signal deterioration. Specifically, in this known interface circuit, once a digital sound signal contains unwanted components such as jitters, noises, or ripples in its waveform, the consequently reproduced analog sound signal will be deteriorated by these unwanted components. The deteriorated analog sound signal degrades tone quality and fidelity in sound reproduction.